Many industrial applications require complex electronic circuits to perform multiple functions which range from the processing of signals, whether in the analog, or the digital form, to the amplification of such signals and the generation of power signals for controlling and powering electric loads, e.g. motors, relays, lamps, etc. Certain applications involve data storage functions as well.
A trend toward ever more compact circuits has led to the development of processes for forming integrated circuits comprised of a large variety of electronic components, which may have even radically different structural and functional features, on a common substrate of semiconductor material. Thus, monolithic integrated structures comprising CMOS (Complementary Metal Oxide Semiconductor) circuits have been developed essentially for processing digital signals, as have bipolar circuits essentially for amplifying analog, signals, DMOS (Diffused Metal Oxide Semiconductor) power components, and bipolar power components for generating and controlling high voltages and large currents. For any data storage functions, however, the prior art provides external devices purposely designed and constructed to perform such functions. It is a widespread opinion among the designers of semiconductor devices that the manufacturing processes of storage devices are too fundamentally different from the manufacturing processes of so-called mixed integrated circuits of the type described above, and that while memory cells may be integrated, at least in theory, with mixed integrated circuits, this would involve in practice the addition of a number of processing steps to the already complicated manufacturing process, thereby making the process a highly critical one and the end product generally unreliable.